The present invention relates to a process for the preparation of 1,3-propanediol by the hydrogenation of hydroxypropionaldehyde (HPA) in aqueous solution on a fixed bed catalyst. In a further aspect, the invention also concerns a catalyst used for the hydrogenation of HPA.
1,3-Propanediol has many different possibilities of application as a monomer unit for the formation of polyesters and polyurethanes and as starting material for the synthesis of cyclic compounds. Various processes are known for the preparation of 1,3-propanediol. These either start from a molecular structure of a C.sub.2 - and C.sub.1 -unit or from a C.sub.3 -unit such as acrolein. When acrolein is used, it is first hydrated in the presence of an acid catalyst to form hydroxypropionaldehyde. The aqueous reaction mixture formed in the process of hydration still contains about 8% of oxaheptanedial in addition to about 85% of HPA and other organic components in minor proportions by weight after removal of unreacted acrolein. This reaction mixture is hydrogenated in the presence of hydrogenation catalysts to produce 1,3-propanediol.
According to U.S. Pat. No. 2,434,100, catalysts containing one or more hydrogenation active metals such as Fe, Co, Ni, Cu, Ag, Mo, W, V, Cr, Rh, Pd, Os, Ir or Pt are suitable for the hydrogenation of HPA to 1,3-propanediol.
As described in German patent 39 26 136.0, the catalyst may be used as such in suspended form or bound to a carrier or form part of fixed bed catalysts. Homogeneous catalysts may also be used. Raney nickel, which may be doped with various other catalytically active metals, platinum on active charcoal, and platinum on aluminum oxide are known as suspension catalysts (from U.S. Pat. No. 3,536,763). A high volume/time yield of hydrogenation is obtained if the solution to be hydrogenated is at a pH of from 2.5 to 6.5, the hydrogenation temperature is in the region of from 30.degree. to 180.degree. C., and hydrogen pressures of from 5 to 300 bar are employed.
Nickel catalysts are mainly used for hydrogenation. Among these, fixed bed catalysts are preferred as they do not need to be filtered off after hydrogenation. Nickel on Al.sub.2 O.sub.3/SiO.sub.2 is an example of a typical fixed bed catalyst for this purpose.
Catalytic hydrogenation entails the risk of small quantities of the catalytically active element being discharged with the stream of product in the form of soluble compounds so that additional operating steps are then necessary to remove these impurities. This phenomenon is most marked in the case of suspension catalysts such as Raney nickel, but nickel fixed bed catalysts also entail the risk of contamination of the product with nickel compounds, albeit in very small quantities.
Hydrogenation processes may be characterized by the conversion rates, selectivities and volume/time yields obtainable by these processes. The conversion rate indicates how many mols of the educt (in this case HPA) are converted into other substances by hydrogenation. The figure is usually given in percent of the mols of educt put into the process: ##EQU1## The selectivity of the hydrogenation process, on the other hand, is a measure of the number of mols of converted educt which are converted into the desired product: ##EQU2## For continuous hydrogenation processes the volume/time yield is another important characteristic which indicates the quantity of product obtainable per unit time and volume of reactor.
In large scale technical hydrogenation of HPA to 1,3-propanediol it is important for the economical efficiency of the hydrogenation process and the quality of the product that the conversion rate and selectivity should be close to 100%. Although the water present in the stream of product as well as residues of HPA and by-products are removed from the propanediol by distillation after hydrogenation, this distillative separation is rendered very difficult by the residue of HPA and by-products and may even become impossible due to reactions between the HPA residue and propanediol to form acetals, whose boiling point is close to the boiling point of propanediol. The lower the conversion rate and the selectivity, the poorer the quality of product obtainable.
Conversion rate, selectivity and volume/time yield are influenced by the properties of the catalyst and by the conditions of hydrogenation such as the reaction temperature, the hydrogen pressure, and the length of hydrogenation time, or, in the case of continuous hydrogenation, by the liquid hourly space velocity.
When HPA is hydrogenated to propanediol, it is observed that the main reaction has a linear relationship to the hydrogen pressure and the time (liquid hourly space velocity in the case of continuous processes), whereas the reaction temperature has hardly any influence.
The formation of by-products, on the other hand, is exponentially dependent upon the temperature. Other conditions being equal, the formation of by-products is doubled with every 10.degree. C. rise in temperature, with the result that the reaction becomes progressively less selective. An increase in the hydrogen pressure, on the other hand, has a positive effect on the selectivity. However, the positive influence of pressure on the selectivity is less powerful than the negative effect of a rise in temperature since the hydrogen pressure increases the velocity of the main reaction only linearly while an increase in temperature increases the velocity of the side reaction exponentially.
One important criterion of the quality of the catalysts used for the hydrogenation process is their service life in operation, i.e., good catalysts should ensure a constant conversion rate and selectivity in the hydrogenation of HPA to propanediol in the course of the operating time. In this respect hydrogenation processes known in the art, in particular those based on nickel catalysts, are found to have insufficient long term stability. As a result, more frequent changes in the whole catalyst package are required, with the well-known attendant problems of elimination of impurities and working up of the compounds containing nickel.